1. Field of the Invention
The present invention relates generally to an ultrasonic motor having a vibrator having a piezoelectric element to vibrate for frictionally driving a moving member, and to an electronic appliance using such an ultrasonic motor. More particularly, the invention relates to an ultrasonic motor to be driven through self-oscillation by utilizing the ultrasonic motor as a vibrator.
2. Description of the Prior Art
In recent years, attention has been drawn to ultrasonic motors for use as actuators based on a new principle in various electronic appliances. Such applications have included a camera auto-focus drive and other various fields. The ultrasonic motor generally adopts a separately excited drive scheme in which a frequency signal created by an external oscillator circuit is applied to a piezoelectric element to cause an oscillation mode on a vibrator having the piezoelectric element. However, the separately excited drive scheme has the defect of being a complicated circuit configuration. In place of the separately excited drive scheme, a self-oscillation drive scheme has been tried and placed into practical use wherein an ultrasonic motor is utilized as a vibrator to cause oscillation due to resonance of the vibrator. The utilization of such a circuit contributes to size reduction and simplification of the circuit and ultimately to reducing the size and cost for an apparatus on which a drive circuit is to be mounted.
However, the self-oscillation drive scheme often undergoes unstable oscillation despite its feature of circuit size reduction and simplification through use of a reduced number of components. The self-oscillator circuit structurally utilizes the resonance of a vibrator as a mechanical filter, thereby amplifying only a particular frequency of signals for sustaining oscillation. However, the vibrator has a plurality of natural oscillation frequencies existing thereon. This might result in a fear of abnormal oscillation in which oscillation occurs at a resonant point including a different vibration mode from a target vibration mode. Such abnormal oscillation is likely to occur where other vibration modes are present in the vicinity of the target vibration mode. Meanwhile, the performance of an ultrasonic motor, such as rotation speed and torque, is largely dependent upon the drive frequency. For the self-oscillation drive scheme, the frequency is given by oscillation caused due to the vibrator and circuit with circuit elements. This results in the possibility that the frequency will be changed and hence change motor performance. Particularly where self-oscillation driving is caused by utilizing a plurality of different vibration modes, the overall or overlapped resonant characteristic is complicated and likely to cause oscillation frequency change and hence abnormal oscillation because of a different resonant frequency and characteristic between the different resonant frequency and characteristic between the different vibration modes.
The present invention is used to drive an ultrasonic motor by a self-oscillation drive circuit. In securing oscillation stability, self-oscillation is caused using a frequency range in which any one only is operable of a plurality of resonant points. For example, where an unwanted resonant point exists in the vicinity of a target resonant point, the unwanted resonant point is raised higher than a resonant point possessed by a vibration mode to be used to operate the ultrasonic motor. Due to this, the effect of an unwanted vibration mode is suppressed as much as possible. Furthermore, self-oscillation is caused using a frequency range where phase change occurs only at the target resonant point. Particularly where a second resonant point exists between a first resonant point and an anti-resonant point thereof, resonance is caused between the first resonant point and the second resonant point. Meanwhile, where the ultrasonic motor is driven using a plurality of different resonant points, self-oscillation driving is made at a higher frequency than any of these resonant points.